A thin jet of fluid impinging on a bath of the same fluid can behave in many different ways. The jet can coil up like a rope. It can float on the surface or plunge air into the fluid bath. This research examines a jet which rebounds off a bath of the same fluid.

Figure 1. A falling liquid stream rebounds off a deep bath of the same silicone oil.

Normally a liquid stream colliding with a pool of liquid merges immediately upon contact, perhaps also bringing air into the pool with it. However when the pool is moving as the stream hits, it can slide along the surface being separated from the pool by a thin layer of air. The air layer supports the jet and lubricates the motion between it and the bath. The same process happens when sliding a piece of paper across a desk or when a car hydroplanes on a wet road. But instead of a hard surface like the desk or the road, the jet is on top of a liquid surface, which is flexible like a trampoline. Because of the weight of the jet and the force required to change directions, the surface is pressed downward and a dent is formed in the shape of a bowl. The sliding jet then ramps out of this bowl and into the air.

The bouncing jet is a new example of steady noncoalescence and a new example of a fluid flow with multiple stable states at the same conditions.

Motivation

The beginning of the project was motivated by curiosity about this unexpected behavior of commonplace material. And because the bouncing jet is visually appealing and counterintuitive. We first observed this phenomena by accident slowly pouring oil from one container to another. We built an apparatus to allow the jet to bounce for longer times, so that we could record its motion.

Relevance to Industrial Processes

Understanding the bouncing jet could benefit the design and operation of these various fluid applications by combining the understanding of many separate phenomenon such as air entrainment, lubrication layers, jet bending, and force balances between inertia, surface tension, and viscous forces.

While a stream of liquid is bouncing, some surrounding air is entrained into a thin layer that separates the jet from the bath. If the air layer breaks into bubbles, the jet stops bouncing. This situation is very common in many industrial processes. In many types of mold casting, molten material is poured into a mold. Often air bubbles are not desirable for reasons ranging from structural integrity to aesthetics.

In industrial processes, the plunging jet is used for mixing two liquids or a gas and a liquid. The swirling motion created by the plunging jet stirs the two liquids together. Chemical agents are often mixed by plunging a jet of the solution into the solution bath. This plunging jet can entrain air, which can be crucial or devastating depending on the intended reaction. Entrained air increase the rate that gas is dissolved into the liquid. In ponds and swimming pools, aeration and mixing can control the levels of nitrogen and oxygen. In metallurgical procedures, gases can react with the molten metal, so the method of pouring or mixing can change the final product.

The air between the jet and the bath separates the two bodies of liquid long enough to give the jet time to bounce. Many fluid bearings are designed to do the same thing: prevent the approach and collision of two bodies using a thin layer of fluid. On the other hand, sometimes separation is not desired. If a jet of oil is sprayed on a surface to cool it, any unintentional deflection or bouncing of the jet could cause heat damage or an explosion. Research on the bouncing jet is in the early stages, so it is difficult to project its uses. Possibly, the bouncing jet could be the basis for a new technique to control the amount of gas entrained into a liquid bath or to control how a liquid stream interacts with another surface.

Setup

The apparatus allows us to create stable bouncing jets reliably. The major control parameters are the flow rate, bath viscosity, falling height, and bath velocity. The purpose of the experiments were two-fold. First, the experiments were designed to find the range of parameters where a bouncing jet is stable. Second, the experiments were to allow a close inspection of the geometry and behavior of the bouncing jet to help deduce the force balance and bouncing mechanism.

The fluid was contained on a rotating table in an acrylic tank. A high speed CCD camera imaged the bouncing jet. The images were read out to a computer. The fluid flow rate was controlled with a pump.

Figure 2. A schematic of the bouncing jet apparatus. The bath of silicone oil sits on a rotating table. A pump creates the falling stream. A camera images the jet in the lab frame.

Results

This study showed that a fluid jet can bounce off of a moving fluid surface stably for a wide range of parameters. We have observed the bouncing jet systematically for silicone oils from 50 to 500 mPa s (56 to 560 times more viscous than water), a jet velocity of 38 to 170 cm/s, a jet diameter from 0.05 to 0.12 cm, and a horizontal bath velocity from 0.5 to 35 cm/s. We also have preliminary observations for silicone oils as low as 10 mPa s.

One factor crucial for bouncing is that the very thin air layer cannot become unstable and break into air bubbles. Also the bath surface must be tense enough to support the jet’s weight and the force of the jet changing directions.

Figure 3: We can follow a fluid parcel sequentially through (1) the jet’s initial stages of falling, (2) its separation from the bulk fluid by a thin layer of air, (3) its bending upwards, and (4) its bounce and subsequent flight. Keep in mind that the bath is moving to the right in this picture.

In the course of conducting this work, there were many exciting and intriguing observations that could not be given the close attention they deserve. Please refer to the Master’s thesis for a compilation of these; a PDF is available below.

Movies

A Newtonian liquid jet can bounce off the surface of a moving bath (see the references below). In the first movie clip, the jet bounces spontaneously as the flow rate decreases rapidly (playback is slowed 8.3 times). The jet and the bath do not mix while bouncing; a thin layer of air separates the jet and the bath. The subsequent sequences show that the jet can bounce twice, that the jet bounces more vertically with smaller bath velocity, and that the bouncing jet can be unsteady but stable. In the last movie clip, the bath is stationary and the bouncing stream is moving. This experiment can easily be done at home by pouring mineral oil into a pan of the same oil.

For the sequence of images showing how a stream is made to bounce twice, the conditions are as follows: the liquid viscosity mu = 106 mPa s, the jet flow rate Q = 0.25 cm^3/s, the jet fall height H = 5.0 cm, and the bath velocity V_{bath} = 18.8 cm/s. The conditions for the movie of the vertically rebounding, unsteady bouncing jet were mu = 361 mPa s, Q = 0.24 cm3/s, H = 4.2 cm, V_{bath} = 0.74 cm/s, the jet rebounds on average 1.1 cm, and the movie plays at 16.7 times slower than real time. For the slow motion movie of the jet poured by hand, the viscosity was mu = 361 mPa s. For the conditions of all other visuals, see the associated Physics of Fluids article below.

How to create the bouncing jet at home

The bouncing jet phenomenon can be observed in many household fluids such as canola oil or heavy mineral oil. Bouncing was first observed in our laboratory while pouring silicone oil by hand into a dish for storage. The materials needed for observing a bouncing jet are simple: a dish (preferably transparent like a glass pie pan, at least 15 cm in diameter and 4 cm tall), a cup, and a small rod (e.g. a cable tie or a chopstick). We measured the viscosity of canola oil and heavy mineral oil (at 22 degrees C) at high shear rates and found both oils to be Newtonian to a good approximation: for canola oil, mu = 65 mPa s at low shear and 4% lower at a shear of 10^4 s^{-1}; for heavy mineral oil, mu = 180 mPa s at low shear and 18% lower at a shear rate of 10^4 s^{-1}.

To observe a bouncing jet, use a dish with liquid about 4 cm deep and pour a thin stream of the liquid (0.5 to 1 cm^3/s) from a cup 3 to 6 cm above the surface. While pouring, move the stream in a circular motion around the dish once about every 2 seconds at a distance 3 to 6 cm from the center. Watch for the jet to bounce while varying the pouring rate, the relative horizontal velocity between the jet and the bath, and the pouring height. To encourage bouncing, pass the small rod through the jet intermittently. A rotating platform (e.g. a record turntable or a Lazy Susan) can be used to rotate the dish instead of moving the cup. If the surface is dirty, clean the surface by stirring the bath or scraping the surface. Blow air on the surface to pop bubbles on the surface. To achieve a very small pouring height, pour the liquid down the rod.

With practice, a jet poured by hand can bounce stably for tens of seconds at a time. Bouncing is also easy to observe in non-Newtonian fluids such shampoo, multigrade motor oil, and concentrated mixtures of liquid soap and water, but the mechanism by which they bounce may not be the same as for Newtonian fluids.

Viscous liquids that bounce are common, so the bouncing jet can be used in the classroom or teaching laboratory to demonstrate and explain fluid dynamics, force balances, and other physics principles. Because it is easy to reproduce and it exhibits many varied behaviors, the bouncing jet can be a playground for the study of fluid jets, their different behaviors, and their stability and instability. Hobbyists, science teachers, and students can study the phenomenon. Students can research one of the many open problems as a science fair project, as a undergrad research project, or as a graduate research project.